There has been known an image forming apparatus in which a plurality of image forming units are arranged in parallel along a sheet conveyance belt and respective color images are sequentially formed on a sheet conveyed on the belt from the image forming units. This kind of image forming apparatus employs a technique referred to as registration so as to suppress deviations of image forming positions of the respective colors with respect to the sheet between the respective image forming units (for example, see JP 2008-225192A).
The image forming apparatus employing the technique includes an optical sensor having a light emission unit and a light reception unit, illuminates light on the belt by the light emission unit and receives the reflected light in the light reception unit, and the light reception unit outputs a light receiving signal corresponding to an amount of the received light. When executing the registration, the image forming apparatus forms marks on the belt by the respective image forming units and reads differences between reflectivity or amounts of reflected lights of a belt surface and mark surfaces based on the light receiving signal from the light reception unit, thereby determining positions of the marks and correcting image forming positions based on a result of the determination.